The "magnetoresistance" (MR) of a material is the resistivity .rho.(H) of the material in an applied magnetic field (H) minus the resistivity .rho..sub.o of the material in the absence of the applied field. This difference .DELTA.p is typically normalized (e.g., by dividing by .rho.(H)) and expressed as a magnetoresistance ratio in percent.
Conventional materials (e.g., PERMALLOY) typically have a positive MR ratio of a few percent. Recently, relatively large values of MR ratio were observed in metallic multilayer structures, e.g., Fe/Cr or Cu/Co, e.g., as high as 40% at room temperature. See, for instance, E. F. Fullerton et. al., Applied Physics Letters, Vol. 63 (12), p. 1699, (1993), and S. S. P. Parkin et. al., Physical Review Letters, Vol. 66(16), p. 2152 (1991). High MR ratios at low temperature (typically below 50K) were also observed in Eu.sub.1-x Gd.sub.x Se. Values of MR substantially larger (in absolute value) than those of conventional materials are usually referred to as "giant" MR. See, for instance, P. M. Levy, Science, Vol. 256, p. 972 (1992), and J. Heremans, J. Physics D:Applied Physics, Vol. 26, p. 1149 (1993).
More recently still, giant MR ratios were observed in mixed metal oxides. K. Chahara et. al., Applied Physics Letters, Vol. 63(14), pp. 1990-1992 (1993), disclose (.DELTA..rho./.rho.(H)) of about -110% in La.sub.0.72 Ca.sub.0.25 MnO.sub.x at about 200K. The material exhibited essentially zero MR at room temperature.
R. von Helmolt et. al., Physical Review Letters, Vol. 71 (14), pp. 2331-2333 (1993) report observation of a room temperature MR ratio of about -150% in thin films of perovskite-like La.sub.0.67 Ba.sub.0.33 MnO.sub.x. As-deposited films were paramagnetic, but after subsequent heat treatment (900.degree. C., air, 12 hours) the samples exhibited a ferromagnetic magnetization curve. Bulk samples of that composition are known to be metallic ferromagnets, with Curie temperature of 343K, but small MR ratio.
U.S. patent application Ser. No. 08/154,766, filed Nov. 18, 1993 by S. Jin et. al., and incorporated heroin by reference, discloses mixed metal oxides that can exhibit a MR ratio of 200% or more (in absolute value) at room temperature. These very high MR ratios were typically attained at relatively high fields, e.g., at 6 Tesla. The prior application also discloses a method of making such material.
Although a high MR ratio is a generally desirable property of a magnetoresistive material, for many potential applications it would be desirable for the material to exhibit a significant change in resistivity in response to a change in the applied field at low values of applied magnetic field, e.g., at field values of 0.05 Tesla (i.e., 500 Oe) or less, since such fields can be readily provided by inexpensive and simple means. Furthermore, it would be highly desirable for the material to exhibit this significant change in resistivity at room temperature (25.degree. C. or 298K), thereby avoiding the need for costly and inconvenient cooling apparatus. This application discloses such materials.